A supercharged internal combustion engine may be used as a motor vehicle drive unit. Within the context of the present disclosure, the expression “internal combustion engine” encompasses diesel engines and Otto-cycle engines and also hybrid internal combustion engines, which utilize a hybrid combustion process, and engines which may be used in hybrid drives which comprise not only the internal combustion engine but also an electric machine which can be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
Due to increasingly stringent emissions standards, it is sought to minimize fuel consumption and reduce pollutant emissions.
Fuel consumption may be inefficient in particular in the case of Otto-cycle engines that is to say in the case of spark-ignition internal combustion engines. The reason for this lies in the principle of the working process of the traditional Otto-cycle engine, in which the desired load or power is set by varying the charge of the combustion chamber, that is to say by means of quantity regulation. By adjusting a throttle flap which is provided in the intake system, the pressure of the inducted air downstream of the throttle flap can be reduced to a greater or lesser extent. For a constant combustion chamber volume, it is possible in this way for the air mass, that is to say the quantity, to be set by means of the pressure of the inducted air. However, quantity regulation by means of a throttle flap has thermodynamic disadvantages in the part-load range owing to the throttling losses.
One approach for dethrottling the Otto-cycle working process is to utilize direct fuel injection. The injection of fuel directly into the combustion chamber of the cylinder is considered to be a suitable measure for noticeably reducing fuel consumption even in Otto-cycle engines. The dethrottling of the internal combustion engine is realized by virtue of quality regulation being used within certain thresholds. By means of direct injection, it is thus possible to realize a stratified combustion chamber charge, which can contribute significantly to the dethrottling of the Otto-cycle working process because the internal combustion engine can be leaned to a very great extent by means of the stratified charge operation, which offers thermodynamic advantages in particular in part-load operation, that is to say in the lower and medium load range, when only small amounts of fuel are to be injected.
The use of an at least partially variable valve drive likewise offers the possibility of dethrottling. A further approach to a solution for dethrottling an Otto-cycle engine is offered by cylinder deactivation, that is to say the deactivation of individual cylinders in certain load ranges. The efficiency in part-load operation can be improved, that is to say increased, by means of a partial deactivation because the deactivation of one cylinder of a multi-cylinder internal combustion engine increases the load on the other cylinders, which remain in operation, if the engine power remains constant, such that the throttle flap may be opened further to introduce a greater air mass into said cylinders, whereby dethrottling of the internal combustion engine is attained overall. During the partial deactivation, the cylinders which are permanently in operation furthermore operate in the region of higher loads, at which the specific fuel consumption is lower. The load collective is shifted toward higher loads.
A further measure for improving the efficiency of an internal combustion engine and/or for reducing the fuel consumption comprises in supercharging of the internal combustion engine, wherein supercharging is primarily a method of increasing power, in which the air demanded for the combustion process in the engine is compressed, whereby a greater mass of air can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and a more expedient power-to-weight ratio. If the swept volume is reduced, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. By means of supercharging in combination with a suitable transmission configuration, it is also possible to realize so-called downspeeding, with which it is likewise possible to achieve a lower specific fuel consumption.
Supercharging consequently assists in the development of internal combustion engines to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
For supercharging, use is often made of an exhaust-gas turbocharger, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is fed to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor conveys and compresses the charge air fed to it, as a result of which supercharging of the cylinders is obtained. A charge-air cooler is advantageously provided in the intake system downstream of the compressor, by means of which charge-air cooler the compressed charge air is cooled before it enters the at least one cylinder. The cooler lowers the temperature and thereby increases the density of the charge air, such that the cooler also contributes to improved charging of the cylinders, that is to say to a greater air mass. Compression by cooling takes place.
The advantage of an exhaust-gas turbocharger in relation to a mechanical supercharger consists in that an exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases, whereas a mechanical supercharger draws the energy desired for driving it directly or indirectly from the internal combustion engine. In general, a mechanical or kinematic connection is used for the transmission of power between the supercharger and the internal combustion engine.
The advantage of a mechanical supercharger in relation to an exhaust-gas turbocharger consists in that the mechanical supercharger generates, and makes available, the desired charge pressure at all times, specifically regardless of the operating state of the internal combustion engine, in particular regardless of the present rotational speed of the crankshaft. This applies in particular to a mechanical supercharger which can be driven by way of an electric machine.
Difficulties may be encountered in achieving an increase in power in all engine speed ranges by means of exhaust-gas turbocharging. A relatively severe torque drop is observed in the event of a certain engine speed being undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. If the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. Consequently, toward lower engine speeds, the charge pressure ratio likewise decreases. This equates to a torque drop.
One such measure, for example, is a small design of the turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a threshold value, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. This approach has the disadvantage that the supercharging behavior is inadequate at relatively high rotational speeds or in the case of relatively high exhaust-gas quantities.
The torque characteristic may also be improved by means of multiple turbochargers arranged in parallel, that is to say by means of multiple turbines of relatively small turbine cross section arranged in parallel, wherein turbines are activated successively with increasing exhaust-gas flow rate.
The torque characteristic of a supercharged internal combustion engine may furthermore be advantageously influenced by means of a plurality of exhaust-gas turbochargers connected in series. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
In particular, with the exhaust-gas turbocharger which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, as a result of which high charge pressure ratios can be obtained even with small compressor flows, which considerably improves the torque characteristic in the lower engine speed range. This is achieved by designing the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which, with increasing exhaust-gas mass flow, an increasing exhaust-gas flow rate is conducted past the high-pressure turbine.
The shifting of the surge limit toward smaller compressor flows is of major significance in the case of supercharged internal combustion engines because it is sought, even in the presence of low charge-air flow rates, to provide charge pressures high enough to thereby realize a satisfactory torque characteristic of the internal combustion engine even at low engine speeds.
In principle, the flow velocity c of the charge air in the intake system decreases significantly in the presence of low charge-air flow rates. According to previous attempts, the approaching-flow velocity, w, relative to the at least one impeller, which rotates at a circumferential velocity, u, is impaired to such an extent that a pressure increase by way of diversion of the charge-air flow as it flows through the impeller can be realized only to an extent, or not at all. Rather, the charge-air flow separates from the impeller blades, a partial backward flow occurs, and the compressor begins to surge.
In one example, the issues described above, as recognized by the inventors herein, may be addressed by a supercharged internal combustion engine having an intake system configured to supply charge air, an exhaust-gas discharge system configured to discharge exhaust gas, and a compressor arranged in the intake system comprising at least one impeller which is arranged on a rotatable shaft in a compressor housing and is equipped with impeller blades, and where the intake system has, upstream of the at least one impeller, a section which runs transversely with respect to a virtual elongation of the shaft of the compressor and in which a pivotable flap is arranged, said section splitting, at least on the side facing away from the at least one impeller, into arcuate ducts which merge so as to form a ring-shaped duct which is of open form on the side facing toward the at least one impeller. In this way, these design measures serve—in particular in the presence of low charge-air flow rates—to advantageously influence the approaching flow to the at least one rotating impeller, and thereby improve the supercharging behavior of the internal combustion engine.
The compressor of the internal combustion engine according to the present disclosure may be a mechanical charger, or else may be the compressor of an exhaust-gas turbocharger.
The internal combustion engine according to the present disclosure comprises, at the inlet side, a pivotable flap which serves for the generation of at least one swirl and which, for this purpose, is arranged, upstream of the at least one impeller, in a section of the intake system which runs transversely with respect to a virtual elongation of the shaft of the compressor.
Said section of the intake system opens into a ring-shaped duct, the arcuate ducts of which run with opposite curvature, preferably extend around the virtual elongation of the compressor shaft, and merge on the side facing away from the section.
The ring-shaped duct is of open form on one side, that is to say on the side facing toward the at least one impeller, wherein the charge air supplied to the ring-shaped duct by the intake system or via the section is supplied to the at least one impeller from the ring-shaped duct via an inlet region of the compressor.
The flap allows charge air to flow past on both sides of the flap, such that it is possible for charge air to be introduced both into one arcuate duct of the ring-shaped duct and into the other arcuate duct of the ring-shaped duct. The charge-air flows conducted through the two ducts have a differently oriented swirl, specifically, on the one hand, a clockwise swirl and, on the other hand, a counterclockwise swirl, or, on the one hand, a swirl in the direction of rotation of the at least one impeller, and on the other hand, a swirl counter to the direction of rotation of the at least one impeller.
By way of suitable pivoting of the flap, the distribution of the charge air between the two arcuate ducts is influenced. That is to say, the charge air can be divided into charge-air flows of different magnitude. Charge-air flows of different magnitude are then conducted to the two ducts of the ring-shaped duct.
One charge-air flow, with its swirl, influences the other charge-air flow, with its differently oriented swirl, and vice versa, that is to say the two charge-air flows of the two arcuate ducts, with differently oriented swirl, influence one another. In particular, the swirl of one charge-air flow diminishes the swirl of the respective other charge-air flow. In this respect, by way of corresponding pivoting of the flap, it is possible to influence not only the orientation of the swirl of the charge-air flow supplied to the at least one impeller, but also the intensity or the extent of said swirl.
The charge-air flow that enters the at least one impeller can thus have imparted to it a velocity component, oriented tangentially with respect to the impeller or with respect to the shaft of the compressor, of different magnitude.
The approaching flow to the at least one rotating impeller is thereby significantly improved, because the absolute velocity, c, of the approaching charge-air flow is rotated relative to the shaft of the compressor such that, in combination with the circumferential velocity, u, of the at least one rotating impeller, a more effectively utilizable relative approaching-flow velocity, w, of the charge air relative to the rotating impeller blades is realized.
The nature and extent of the rotation of the absolute velocity, c, of the approaching charge-air flow relative to the shaft of the compressor can be influenced by pivoting of the flap.
It is possible to dispense with a complex guide device for influencing the approaching flow, which, for example, forcibly imparts a swirl, that is to say a velocity component transversely with respect to the shaft of the compressor or in the circumferential direction, to the charge-air flow. Along with the guide device, the costs for the generally adjustable guide device, and the control thereof, are also eliminated. The problem whereby a guide device provided in the intake system constitutes merely an undesired flow resistance, and reduces the pressure in the charge-air flow, in particular in the presence of high engine speeds or high charge-air flow rates, is likewise eliminated. Dense packaging of the compressor unit as a whole is thus made possible.
An object on which the present disclosure may be based is achieved by means of the internal combustion engine according to the present disclosure, that is to say a supercharged internal combustion engine according to the preamble of claim 1 is provided, the supercharging behavior of which in the presence of low charge-air flow rates is improved.
Embodiments are advantageous in which the flap is pivotable such that all of the charge air can be supplied to substantially only one of the two ducts of the ring-shaped duct. It is preferably possible for all of the charge air to be supplied either to one duct or to the other duct of the ring-shaped duct. The latter makes it possible to realize an intense swirl, specifically both in one direction of rotation and in the other direction of rotation.
The design measures proposed according to the present disclosure are not only suitable for shifting the surge limit toward lower charge-air flow rates and thus for improving the supercharging behavior of the internal combustion engine in the presence of low charge-air flow rates.
By optimizing the approaching-flow conditions of the at least one impeller, the efficiency of the compressor can be fundamentally improved, and thus the supercharging behavior of the internal combustion engine can be improved under all operating conditions, in particular also in the presence of medium and relatively high charge-air flow rates.
Embodiments of the supercharged internal combustion engine are advantageous in which the section intersects the virtual elongation of the shaft of the compressor.
Embodiments of the supercharged internal combustion engine are advantageous in which the section runs perpendicular to the virtual elongation of the shaft of the compressor.
The two above embodiments relate to the arrangement or the orientation of the relevant section in the intake system in which the pivotable flap is arranged.
The embodiments facilitate the formation of a ring-shaped duct which runs coaxially with respect to the impeller or with respect to the virtual elongation of the shaft. Thus, the formation of a swirling flow, that is to say an approaching charge-air flow with swirl, is simplified.
Here, embodiments of the supercharged internal combustion engine are advantageous in which the pivotable flap is arranged substantially centrally in the section, such that charge air can be conducted past on both sides of the flap.
The central arrangement of the flap allows charge air to flow past on both sides of the flap, and thus may split up the charge air between the two arcuate ducts of the ring-shaped duct and form charge-air flows with differently oriented swirl. By way of suitable pivoting of the flap, the charge air is split up between the two ducts, or charge-air flows of different magnitude with differently oriented swirl are generated.
Embodiments of the supercharged internal combustion engine are also advantageous in which the flap is pivotable about an axis which runs substantially parallel to the shaft of the compressor. This embodiment facilitates the formation of a swirling flow which runs coaxially with respect to the shaft of the compressor impeller, that is to say an approaching charge-air flow with a swirl around the virtual elongation of the shaft.
Embodiments of the supercharged internal combustion engine are advantageous in which the ring-shaped duct is, at least in sections, of circular form. Said at least partially circular form of the ring-shaped duct advantageously corresponds to the circular form of the at least one rotating impeller. An at least partially rotationally symmetrical form of the ring-shaped duct is suited to the rotation of the at least one impeller. Here, the ring-shaped duct is responsible for at least the generation of the swirl or of the charge-air flow with swirl.
For the stated reasons, embodiments of the supercharged internal combustion engine are also advantageous in which the ring-shaped duct runs around the virtual elongation of the shaft of the compressor.
Embodiments of the supercharged internal combustion engine are advantageous in which the ring-shaped duct is arranged spaced apart from the at least one impeller.
Embodiments of the supercharged internal combustion engine are advantageous in which an inlet region of the compressor is arranged between the ring-shaped duct and the at least one impeller.
In this context, embodiments of the supercharged internal combustion engine are advantageous in which the inlet region runs and is configured coaxially with respect to the shaft of the compressor, such that the charge air can also be supplied to the compressor impeller substantially axially. The charge air then does not have to be deflected while flowing through the inlet region, in order to be fed axially to the compressor. Since a deflection or directional change of the charge air flow is absent in the inlet region, unnecessary pressure losses in the charge air flow as a consequence of flow deflection are avoided. The degree of efficiency and the charge pressure ratio can be increased.
Embodiments of the supercharged internal combustion engine are advantageous in which a throttle device is arranged in the intake system downstream of the compressor.
It may be expedient for a throttle device to be provided in the intake system in order, in the context of quantity regulation, to be able to adjust the load in wide ranges, in particular in the presence of very low charge-air flow rates, or in order to be able to shut off the supply of air to the cylinders.
In this context, embodiments of the supercharged internal combustion engine are advantageous in which the throttle device is a throttle flap.
Embodiments of the supercharged internal combustion engine are advantageous in which a charge-air cooler is arranged in the intake system downstream of the compressor. The temperature of the charge air is reduced by way of cooling and the density is increased in this way. Compression by cooling takes place. In this way, the cooler contributes to improved charging of the cylinders.
Embodiments of the supercharged internal combustion engine are advantageous in which the compressor is an axial compressor, in which the exit flow runs substantially axially. In the context of the present disclosure, “substantially axially” means that the speed component in the axial direction is greater than the radial speed component.
Embodiments of the supercharged internal combustion engine are likewise advantageous in which the compressor is a radial compressor. This embodiment offers advantages in particular with regard to dense packaging if the at least one compressor is the compressor of an exhaust-gas turbocharger. The compressor housing may be configured as a spiral or worm housing.
Embodiments of the supercharged internal combustion engine are advantageous in which an exhaust-gas turbocharger is provided which comprises a turbine arranged in the exhaust-gas discharge system and a compressor arranged in the intake system, the turbine and the compressor being arranged on the same rotatable shaft.
In this context, embodiments of the supercharged internal combustion engine may be advantageous in which the compressor is the compressor of the exhaust-gas turbocharger.
Embodiments of the supercharged internal combustion engine may also be advantageous in which the compressor is a mechanical charger.
Embodiments of the supercharged internal combustion engine are advantageous in which an exhaust-gas recirculation arrangement is provided.
In this context, embodiments of the supercharged internal combustion engine are advantageous in which an exhaust-gas recirculation arrangement is provided which comprises a line which opens into the intake system downstream of the compressor.
To adhere to future threshold values for nitrogen oxide emissions, use may be made of exhaust-gas recirculation, that is to say a recirculation of exhaust gases from the exhaust-gas discharge system into the intake system, wherein the nitrogen oxide emissions can be lowered considerably with increasing recirculation rate.
Here, embodiments are advantageous in which a cooler is provided in the line to the exhaust-gas recirculation arrangement, which cooler lowers the temperature in the hot exhaust-gas flow and thus increases the density of the exhaust gases. The temperature of the cylinder fresh charge which results upon the mixing of the fresh air with the recirculated exhaust gases is reduced in this way, as a result of which said cooler also contributes to improved charging of the combustion chamber with charge air.
Embodiments are advantageous in which a shut-off element is provided in the line for exhaust-gas recirculation. Said shut-off element serves for the control of the exhaust-gas recirculation rate.
Another object on which the present disclosure may be based, specifically that of specifying a method for operating a supercharged internal combustion engine of a type described above, is achieved by way of a method wherein the flap arranged in the intake system upstream of the at least one impeller is pivoted to influence an approaching-flow angle α of the charge air supplied to the compressor relative to the impeller blades of the at least one impeller.
That which has already been stated with regard to the internal combustion engine according to the present disclosure also applies to the method according to the present disclosure, for which reason reference is generally made at this juncture to the statements made above with regard to the internal combustion engine according to the present disclosure. The different internal combustion engines demand, in part, different method variants.
Embodiments of the method are advantageous in which the charge air supplied to the less-flow angle α is improved.
The flap imparts a velocity component oriented tangentially with respect to the impeller or with respect to the shaft of the compressor, that is to say a swirl, to the charge-air flow entering the impeller, whereby the compressor can compress even relatively low charge-air flow rates without the risk of surging.
Here, embodiments of the method are advantageous in which the charge air supplied to the compressor has a swirl forcibly imparted to it using the flap when the engine speed of the internal combustion engine nmot falls below a predefinable engine speed.
The charge-air flow rate basically increases with the engine speed nmot. In a traditional Otto-cycle engine with quantity regulation, the charge-air flow rate increases with increasing load even at a constant engine speed, whereas in a traditional diesel engine with quality regulation, the charge-air flow rate is, as a first approximation, dependent merely on engine speed, because in the event of a load shift at constant engine speed, the mixture composition but not the mixture quantity is varied.
The internal combustion engine according to the present disclosure is a supercharged internal combustion engine, such that consideration may also be given to the charge pressure on the intake side, which may vary with the load and/or the engine speed and which has an influence on the charge-air flow rate. The relationships discussed above regarding the charge-air flow rate and the load or engine speed consequently apply only conditionally in this general form. It may therefore be advantageous for consideration to be given primarily on the charge-air flow rate and not directly to the engine speed.
Embodiments of the method are advantageous in which the charge air supplied to the compressor has a swirl forcibly imparted to it using the flap when the load of the internal combustion engine Tmot falls below a predefinable load. In the case of quantity regulation, the charge-air flow rate increases with increasing load, even in the case of a constant engine speed.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.